Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for illuminating a desired target during a seismic survey performed with stationary seismic sources.
Discussion of the Background
Reflection seismology is a method of geophysical exploration to determine the properties of a portion of a subsurface layer in the earth, information that is especially helpful in the oil and gas industry. Marine reflection seismology is based on the use of a controlled source that sends energy waves into the earth. By measuring the time it takes for the reflections to come back to plural sensors, it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
For marine applications, sources are mainly impulsive (e.g., compressed air is suddenly allowed to expand). One of the most used sources is air guns which produce a high amount of acoustic energy over a short time. Such a source is towed by a vessel either at the water surface or at a certain depth. Acoustic waves from the air gun propagate in all directions. The emitted acoustic waves' typical frequency range is between 6 and 300 Hz. However, the frequency content of the impulsive sources is not fully controllable, and different sources are selected depending on a particular survey's needs. In addition, use of impulsive sources can pose certain safety and environmental concerns.
Thus, another class of sources that may be used are vibratory. Vibratory sources, including hydraulically-powered, electrically-powered or pneumatically-powered sources and those employing piezoelectric or magnetostrictive material, have been used in marine operations. However, there is no large-scale use of such sources because they have limited power and are unreliable due to the number of moving parts required to generate seismic waves. A positive aspect of vibratory sources is that they can generate signals that include various frequency bands, commonly referred to as “frequency sweeps.” In other words, the frequency band of such sources may be better controlled, as compared to impulsive sources.
One example of such a vibratory source is described in U.S. patent application Ser. No. 13/415,216 (herein the '216 application), filed on Mar. 8, 2012, and entitled, “Source for Marine Seismic Acquisition and Method,” assigned to the same assignee as the present application, the entire content of which is incorporated herein by reference.
Recent trends in the field of seismic exploration suggest that techniques for conducting 4-dimensional (4D) surveys to image known hydrocarbon reservoirs are in high demand. 4D seismic surveys are 3D surveys that are repeated, for example, several times a year. By comparing images generated by each 3D survey over time, the differences in successive surveys may be analyzed to help provide useful information to petroleum engineers so they may adjust injection or pumping schedules to facilitate efficient extraction of hydrocarbon deposits. For land exploration, one such system developed by the assignee of this patent application is called Seismovie™ surveying. In Seismovie surveying, a plurality of piezoelectric sources is buried at a depth sufficient to ensure that source coupling remains invariant over time. Also, the piezoelectric sources are buried deep enough so that factors like soil moisture and others are not a problem. Geophones and/or hydrophones are also buried for 4D land monitoring. Permanent installation of sources and sensors helps to ensure survey repeatability so that changes to the reservoir should be the only variable, resulting in easier comparisons over time.
However, the situation is different in the marine environment. Typically, neither the source array nor the sensors are fixed. In some situations, the sensors are located on the ocean floor, e.g., ocean bottom cables or ocean bottom nodes. A traditional marine acquisition system 100 is illustrated in FIG. 1. System 100 includes a vessel 110 configured to tow a source array 120 and one or more streamers 130. Source array 120 may include a low-frequency array 122 and a high-frequency array 124 towed at different depths. Vessel 110 may include a controller 112 configured to actuate the source array and/or receive seismic data from sensors 132 distributed along streamer 130. Controller 112 may be configured to have other capabilities, for example, to interact with the navigation system, to control various birds 134 distributed along the streamer. Streamer 130 may have various shapes, i.e., linear, curved, variable-depth, etc. Controller 112 may also be configured to command other position control devices not shown in FIG. 1 that are associated with the source array.
During a seismic survey, vessel 110 follows a pre-determined path to cover the survey area of interest. Source elements of the source array are fired at pre-determined positions and moving sensors record reflected and/or refracted seismic waves. However, for a 4D seismic survey, because at least two different 3D seismic surveys are performed over the same area of interest, shooting the source elements at exactly the same locations and recording the seismic signals at the same receiver locations is a challenging task. Note that water currents may be strong, which make the source elements and/or the sensors deviate from their pre-determined positions, ocean waves may keep the vessel from following exactly the pre-determined path, etc.
Thus, there is a need to overcome the above-noted difficulties and to provide a new seismic acquisition data system with good repeatability, irrespective of ambient conditions.